The Clinical Branch of the National Eye Institute asked us to develop an electroretinogram (ERG). This instrument records the field electrical potential generated by the retina and elicited by a visual stimulus. Electroretinography is a noninvasive technique used in the diagnosis and management of diseases of the retina. To elicit an ERG, an appropriate stimulator must be used to deliver a visual stimulus to the retina. In clinical electroretinography, this stimulus is usually a brief flash of light with known luminance and spectral composition. The clinically relevant parameters of the ERG depend, among other variables, on the magnitude of the retinal area stimulated by light. The ideal stimulus for most clinical applications illuminates the entire retina uniformly. This requires an extended source of uniform luminous intensity covering the entire visual field. The instrument being designed is called a Ganzfeld (full-field) stimulator; it operates using the principle of an integrating sphere. ERG waveforms vary with the characteristics of the light stimulus and the adaption state of the retina. For this reason, a Ganzfeld dome stimulator should allow the control of luminance and spectral composition of both stimulus and adapting background. A convenient way to achieve this end is to interpose neutral density filters (to vary spectral composition) between the light source and the Ganzfeld dome.